Grimes is renting her campaign bus from her father’s company for a fraction of the normal market price, Politico reports, which is illegal according to Federal Election Commission campaign laws.

Her father Jerry Lundergan, a well-connected Kentucky Democrat, purchased the bus last year, just as her campaign was beginning. His company rented the bus to Grimes for about about $465 a day in June, according to Politico’s review of FEC records.

But officials at four different bus companies told Politico they would charge between $1,500 and $2,000 a day to rent the same type of bus. And McConnell’s campaign said it recently spent more than $2,000 per day on a similar bus.

FEC law states that a campaign cannot receive goods and services below the market price from a corporation — period. So, if that difference in cost is accurate, the thousands of dollars the campaign is saving could be considered an illegal in-kind contribution, FEC expert Larry Noble told Politico.

“[He] can volunteer himself to the campaign,” Noble said, “but what he can’t do is volunteer his company.”

The Grimes campaign told Politico it is paying the proper price for the bus. “In determining the appropriate rate, the campaign obtained costs of comparable providers in the Kentucky and regional market and arrived at a reasonable reimbursement cost,” Grimes’ campaign lawyer Marc Elias said. “We have reviewed the campaign’s methodology and agree that it applies with the applicable rules.”

If the FEC finds the Grimes campaign at fault, it could be forced to pay a civil fee.